Add Issue Triage and Backlog Management Guidelines

Summary

This MR documents the systematic approach for triaging issues and managing the Package stage backlog, based on the comprehensive refinement completed in issue #[issue-number].

What's Changed

Added new "Issue Triage and Backlog Management" section to team handbook
Documented backlog labeling process (backlog::prospective vs backlog::no-commitment)
Established clear guidelines for organizing issues by type (feature, maintenance, bug)
Defined maintenance epic structure and sub-categories
Created repeatable process for epic management and priority assignment

Impact

Issues processed: 1,400+ issues reviewed and organized
Features: 363 → backlog::prospective, 53 → backlog::no-commitment, 387 closed
Maintenance: 800+ → 223 issues labeled backlog::prospective, ~600 closed
Bugs: 222 → 135 issues labeled backlog::prospective

Benefits

Provides clear triage process for future issue management
Ensures consistent labeling and epic assignment
Establishes 24-month planning horizon for backlog commitment
Creates structured maintenance epic framework for better organization